LifecycleObserver监听Activity生命周期

通过LifecycleObserver实现自动计数，退出到后台时，停止计数，当再次回到前台时从上次位置开始计数

1.自定义控件并实现LifecycleObserver

import android.content.Context;
import android.os.SystemClock;
import android.util.AttributeSet;
import android.widget.Chronometer;
import androidx.lifecycle.Lifecycle;
import androidx.lifecycle.LifecycleObserver;
import androidx.lifecycle.OnLifecycleEvent;

/**
 * Chronometer 可以实现自动计数
 */
public class MyChronometer extends Chronometer implements LifecycleObserver {

    private static long lastTime;
    public MyChronometer(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    //监听Activity onPause事件
    @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE) //LifecycleObserver特有的写法
    private void pauseMeter(){
        lastTime = SystemClock.elapsedRealtime() - getBase();
        stop();
    }

    //监听Activity onResume事件
    @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
    private void resumeMeter(){
        setBase(SystemClock.elapsedRealtime()- lastTime);
        start();
    }
}

布局中

 <com.gao.lifecycle.MyChronometer
        android:id="@+id/chronometer"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!"
        android:textSize="30sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

2.在Activity中将控件设置为Activity的生命周期观察者

import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;

public class MainActivity extends AppCompatActivity {

    MyChronometer chronometer;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        chronometer = findViewById(R.id.chronometer);
        //添加chronometer为观察者
        getLifecycle().addObserver(chronometer);
    }
}